The
subject of this award is to contribute to an in-depth understanding of the
initial steps in hydrogen peroxide activation for manganese catalysts by
defining the physical and reactivity properties of peroxomanganese(III)
intermediates, which are proposed to be among the first species formed when
manganese complexes react with hydrogen peroxide. Such catalytic systems are of
interest because manganese centers and hydrogen peroxide mediate important
oxidative transformations, including those involved in the functionalization of
petroleum feedstocks and fabric and paper bleaching. In year two of this
project we have i) published a detailed study comparing geometric and
electronic stuctures of peroxomanganese(III) complexes supported by tetradentate
aminopyridyl and aminoquinolinyl ligands, ii) designed and synthesized Mn(II) complexes
supported by new aminoquinoline ligands expected to be more oxidatively robust
than their first-generation analogues, and iii) elucidated the reaction
landscape of a manganese(II) complex with superoxide and hydrogen peroxide,
which features the conversion of a peroxomanganese(III) adduct to a bis(m-oxo)dimanganese(III,IV) species.

In total, this two-year award has supported the training of three graduate
students and has resulted in three publications to date, positively impacting
my career as an assistant professor.

Figure
1.
Ligands used to support peroxomanganese(III) complexes, representative
structure of a [MnIII(O2)(L7py2R)]+
complex, and correlation of Mn-O(peroxo) distance with lowest-energy MCD
transition.

In this year, we completed our
comparison of MnIII-O2 adducts supported by tetradentate
ligands with systematically varied steric and electronic properties (Geiger, R.
A. et al.Eur. J. Inorg. Chem.2012, 1598-1608.). Magnetic
circular dichroism (MCD) and density functional theory (DFT) studies of these
six complexes establish a correlation between the computed Mn-O(peroxo) bond
length and the position of the lowest-energy MCD band (band 1; see Figure 1).
Notably, this correlation is attributed to the steric properties of the
supporting ligand; electronic perturbations among this series have a limited
effect. For example, pyridine substituents (R = Cl, H, and Me) in the
4-position hardly affect the energy of band 1, whereas steric interactions
between pyridine or quinoline groups (e.g., for the L7py26-Me
and L7q2 ligands) lead to a red-shift of band 1 that is
attributed to an elongation of one Mn-O(peroxo) bond (Figure 1, far right). We
intend to exploit this tuning of the Mn-O(peroxo) interaction to activate MnIII-O2
species using sterically encumbered ligands. These results also suggest that
enzyme active sites may prevent formation of symmetric MnIII-O2
adducts (i.e., with near equivalent Mn-O(peroxo) distances) by putting
strict steric constraints on the peroxo binding pocket. Presumably, asymmetric
MnIII-O2 spcies with an elongated Mn-O(peroxo) bond would
be more reactive towards electrophilic substrates, as previously suggested (Annaraj,
J. et al.Angew. Chem. Int. Ed.2009, 48, 4150-4153).

Our previous work has established that
the peroxomanganese(III) adducts supported by our L7py2R
family of ligands are quite electrophilic (Geiger, R. A. et al.Dalton
Trans.2011, 40, 1707-1715.). However, we have been unable to
trap any intermediates when these compounds are treated with acids or acyl
chlorides. Instead we observe the immediate disappearance of the
peroxomanganese(III) chromophore and the formation of manganese(II) species.
Acids or acyl chloride reagents are expected to react with the MnIII-O2
unit to generate MnIII-OOH or MnIII-OOR species,
respectively, which could subsequently undergo O-O cleavage to yield
high-valent oxomanganese species. Although such reactions have been observed for
corresponding iron(III) complexes, such chemistry has remained quite elusive
for manganese(III) centers. For our complexes, we propose that the
peroxomanganese(III) species reacts with acids or acyl chlorides to yield a
strong oxidant that initiates decomposition of the L7py2R
ligand. In support, Groni et al. have observed that the
peroxomanganese(III) complex [MnIII(O2)(mL52)]+
(mL52 = N-methyl-N,N¢,N¢-tris(2-pyridylmethyl)ethane-1,2-diamine) decays upon
treatment with acid to yield a Mn(II) center bound to two pyridinecarbyoxylate
ligands that are formed by oxidation of mL52 (Groni, S. et
al.Inorg. Chem.2008, 47, 3166-3172). Thus, we
reasonably suspect that the methyl linkers of the L7py2R
ligands could be susceptible to oxidization by high-valent oxomanganese species.
Accordingly, we have synthesized the L7BQ and L8BQ
ligands, where the quinoline rings are attached directly to the 1,4-diazepane
and 1,5-diazacyclooctane fragments, respectively (Figure 2). The crystal
structure of the [MnII(L7BQ)(OTf)2] complex (Figure
2) shows the tetradentate ligand bound in the trans fashion, similar to
that of the L7py2R ligands. We are currently
in the process of characterizing the peroxomanganese(III) complexes supported
by the BQL7 and BQL8 ligands and exploring their
activation by acids and acyl chlorides. We note that the putative [MnIII(O2)(L7BQ)]+
complex shows enhanced thermal stability relative to [MnIII(O2)(L7q2)]+.

Figure
2.
Structures of the L7BQ ligand (left) and an ORTEP diagram of the [MnII(L7BQ)(OTf)2]
complex (right).

In
Year 1 of this project, we described the geometric and electronic structures of
a peroxomanganese(III) unit supported by the pentadentate N4py ligand (N4py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine).
This complex was generated by treatment of [MnII(N4py)(OTf)2]
with KO2 in acetonitrile (Scheme 1). Unlike all other peroxomanganese(III)
complexes reported to date, [MnIII(O2)(N4py)]+
can only be generated using KO2. If [MnII(N4py)(OTf)2]
is treated with H2O2 and Et3N in acetonitrile,
the dinuclear [MnIIIMnIV(m-O)2(N4py)2]3+ complex
is formed even at low temperatures. When [MnIII(O2)(N4py)]+
is formed from [MnII(N4py)(OTf)2] and KO2 in
high yields, it decays to a mixture of manganese(II) species. However, if it is
formed in only 50% yield or less, it converts to the [MnIIIMnIV(m-O)2(N4py)2]3+
complex. Separate experiments have shown that treatment of [MnIII(O2)(N4py)]+
with [MnII(N4py)(OTf)] also leads to the formation of [MnIIIMnIV(m-O)2(N4py)2]3+.
Thus, the MnII-N4py system shows a complex reaction landscape,
featuring a variety of intermediates. Because dissociation of a pyridylmethyl
arm is require for formation of [MnIII(O2)(N4py)]+,
our current hypothesis is that the rate of pyridine dissociation influences
whether or not the MnII center reacts with an oxidant to afford a MnIII-O2
or MnIIIMnIV dimer. Understanding the complex factors
that govern the relative rates of formation of these intermediates is important
given that oxo-bridged MnIIIMnIV dimers are often viewed
as "dead-end" intermediates due to their high thermodynamic stability. Understanding
how to disfavor the formation of such species, could be an important
step in accessing more reactive intermediates.

Scheme
1.
Experimentally observed reaction landscape for [MnII(N4py)(OTf)]+
in the presence of H2O2 or KO2.